Reference signal encoding for seismic while drilling measurement

Abstract

A method for making seismic while drilling (SWD) measurements by determining and analyzing the reference signal downhole near the drill bit and sending a limited quantity of information to the surface by measurement-while-drilling (MWD) telemetry. In one embodiment, a library of anticipated drill bit wavelets is stored in memory downhole and in memory at the surface. The best matching wavelet is identified by the processor downhole and then a code identifying the wavelet and a scaling factor is sent to the surface. At the surface, the best matching wavelet is retrieved based on the code received and then a reconstructed signal is created using the retrieved wavelet and the scaling factor. In another embodiment, key characteristics of the signal such as central frequency, frequency band, etc., are calculated downhole and transmitted to the surface. These key characteristics are then used to reconstruct the reference signal which is then used for correlation of surface detected signals.

What is claimed is: 
     
       1. A data collection method, comprising: (a) in a downhole processor, analyzing an acoustic signal generated by an operating drill bit to derive data therefrom; and   (b) in at least one second processor reconstructing a copy of said acoustic signal from said data, and using said copy as a reference signal, in combination with acoustic information detected at positions separate from said drill bit, to provide seismic characterization.   
     
     
       2. The method of claim 1, wherein said data comprises a code corresponding to a best matching wavelet in a library of wavelets. 
     
     
       3. The method of claim 1 wherein said data comprises a key characteristic of said reference signal. 
     
     
       4. The method of claim 3 wherein said key characteristic is at least one of (i) a central frequency of said acoustic signal, (ii) a frequency bandwidth of said acoustic signal, and (iii) a statistical parameter of said acoustic signal. 
     
     
       5. A seismic-while-drilling method, comprising the steps of: (a.) in a downhole processor, analyzing the acoustic signal generated by an operating drill bit to derive data therefrom;   (b.) transmitting said data to at least one processor located at or near the earth's surface;   (c.) reconstructing a copy of said acoustic signal from said data; and   (d.) using said copy as a reference signal to interpret acoustic information detected at positions separate from said drill bit.   
     
     
       6. The method of claim 5, wherein said data comprises a code corresponding to a best matching wavelet in a library of wavelets. 
     
     
       7. The method of claim 5 wherein said data comprises a key characteristic of said reference signal. 
     
     
       8. The method of claim 7 wherein said key characteristic is at least one of (i) a central frequency of said acoustic signal, (ii) a frequency bandwidth of said acoustic signal, and (iii) a statistical parameter of said acoustic signal. 
     
     
       9. A method of gathering geophysical information, comprising: drilling a well bore with a drill bit which produces seismic energy;   in a down hole processor, analyzing a reference signal generated by said drill bit during continuing drilling operations to determine key characteristics of said reference signal;   transmitting said key characteristic to the surface;   obtaining a reconstructed reference signal at the surface based on said key characteristics;   detecting a seismic signal at the surface; and   correlating said seismic signal with said reconstructed reference signal to obtain information about subterranean geologic formations.   
     
     
       10. The method of claim 9, wherein said key characteristics is at least one of (i) a central frequency of the reference signal, (ii) a frequency bandwidth of the reference signal, and (iii) a statistical parameter of the reference signal. 
     
     
       11. The method of claim 9, further comprising using said information about subterranean geologic formations for determining the presence and location of hydrocarbons. 
     
     
       12. A method of gathering geophysical information comprising: creating a library of anticipated bit wavelets and storing said library in a downhole memory and in a surface memory;   generating a reference signal downhole;   analyzing said reference signal downhole to identify a code corresponding to a best matching wavelet from said library and a scale factor;   transmitting said code and said scale factor to a surface location;   obtaining a reconstructed reference signal at the surface location from said transmitted code and said scale factor using said surface memory;   using said reconstructed reference signal to correlate a detected signal to determine geophysical information.   
     
     
       13. The method of claim 12, wherein said detected signal is recorded using geophones positioned at a surface location. 
     
     
       14. A system for seismic exploration comprising: a downhole processor that analyzes an acoustic signal generated by an operating drill bit to derive data therefrom;   a second processor operatively connected to receive said data; and   at least one acoustic detector remote from said downhole processor; wherein   said second processor is remote from said downhole processor and uses said data to interpret acoustic information received by the at least one acoustic detector.   
     
     
       15. The method of claim 14, wherein the at least one acoustic detector is a geophone located at the earth's surface. 
     
     
       16. The method of claim 14, wherein the at least one acoustic detector further comprises a plurality of spaced apart geophones located at the earth's surface.